1. Field of the Invention
The present invention relates to a power storage device and a method for manufacturing the power storage device. Note that, in this specification, power storage devices refer to part or all of devices each having a function of storing electric power.
2. Description of the Related Art
In recent years, power storage devices such as lithium-ion secondary batteries and lithium-ion capacitors have been developed. Further, it has been proposed that such power storage devices be used in a variety of electric appliances such as a refrigerator (see Patent Document 1), an air conditioner (see Patent Document 2), an indoor lighting device (see Patent Document 3), and a microwave oven (see Patent Document 4).
An electrode for the power storage device is manufactured by forming an active material over a surface of a current collector. When an active material formed over one surface of a current collector has a layer-like shape, it is referred to as an active material layer. Note that the active material layer may contain a material other than the active material (e.g., a conductive additive or a binder). As an active material which occludes lithium, such as graphite or silicon, is used, because these materials can occlude and release ions serving as carriers. In particular, silicon has attracted attention because it has a higher theoretical capacity than graphite and is advantageous in increasing the capacities of power storage devices.
For example, Non-Patent Document 1 discloses a lithium-ion secondary battery formed using whisker-like single crystal silicon as an active material. Non-Patent Document 1 suggests that in the case where whisker-like silicon is used, the structure of an electrode is not easily damaged even when the volume of silicon is changed due to occlusion and release of lithium ions, resulting in increases in charge/discharge characteristics.